This application discloses an invention which is related, generally and in various embodiments, to a system and method for controlling a Modular Multilevel Converter (M2LC) system.
Many papers have been published regarding the Modular Multilevel Converter (M2LC) topology. FIG. 1 illustrates a two-level configuration of an M2LC cell having two terminals, FIG. 2 illustrates a three-level configuration of an M2LC cell having two terminals, and FIG. 3 illustrates a M2LC system.
As shown in FIG. 1, the M2LC cell includes two switching devices, two diodes, a capacitor and two terminals. With the configuration shown in FIG. 1, the two switching devices can be controlled such that one of two different potentials (e.g., zero volts or Vcap) may be present across the two terminals. As shown in FIG. 2, the M2LC cell includes four switching devices, four diodes, two capacitors and two terminals. With the configuration shown in FIG. 2, the four switching devices can be controlled such that one of three different potentials (e.g., zero volts, Vcap, or 2Vcap) may be present across the two terminals. Although other topologies of the M2LC cells are possible, all of the topologies may be defined as two-terminal subsystems or cells with internal capacitor energy storage(s) which are capable of producing various levels of voltages between the two terminals depending on the state of the switching devices.
As shown in FIG. 3, the M2LC system may be configured as a three-phase bridge which includes a plurality of M2LC cells (subsystems), where the M2LC cells are arranged as three output phase modules. Of course, other M2LC systems may be configured differently than shown in FIG. 3. For example, other M2LC systems may be configured as two output phase modules. For the M2LC system of FIG. 3, each output phase module includes a plurality of series-connected M2LC cells, and each output phase module is further arranged into a positive arm (or valve) and a negative arm (or valve), where each arm (or valve) can be separated by an inductive filter. Each output phase module may be considered to be a pole. The respective inductive filters are utilized in the M2LC topology when more than one pole of the M2LC system is paralleled on one common DC bus. The inductive filters operate to reduce currents produced by the switching in the arms of the M2LC system. The spectral content of the arm currents can be shown to be a function of the switch functions and the output current of the pole. Some embodiments of the M2LC system employ inductive filters which have relatively large inductance values along with an active pole current controller to ultimately control the quality of the arm currents.
Although not shown in FIGS. 1-3 for purposes of simplicity, it will be appreciated that each M2LC cell also includes a local controller, and each local controller may be communicably connected to a higher level controller (e.g., a hub controller) of the M2LC system.
It will be appreciated that the M2LC topology possesses the advantages of the Cascaded H Bridge (CCH) topology in that it is modular and capable of high operational availability due to the ability to add one or more redundant cells in each arm. Additionally, the M2LC topology can be applied in common bus configurations. In contrast to M2LC, CCH requires the utilization of a multi-winding transformer which contains individual secondary windings which supply input energy to the cells.
However, unlike CCH, the M2LC cells are not independently supplied from isolated voltage sources or secondary windings. For a given M2LC cell, the amount of energy output at one of the two terminals depends on the amount of energy input at the other one of the two terminals and to some extent the ability of the cell to store and release energy. This can cause a problem in controlling the DC link voltages in these cells during pre-charge of the power circuit or during abnormal operation when one or more of the cells needs to be bypassed or made inactive.
Various methods of balancing the DC link voltages actively from the hub control system of the M2LC system have been employed but such methods require excess sub-system or cell capacity (in the form of extra cells or partially modulated cells). These methods are also relatively complicated in that they require ongoing monitoring of each subsystem link voltage and the use of a complicated sorting system which selects particular cells for modulation depending on their relative DC link voltage and the direction and magnitude of the output current level. Furthermore, these methods tend to perform poorly at low output current levels and frequencies, and require that a load is connected and conducting current in order to supply the needed charge to balance the capacitor voltages.
Additionally, various methods of implementing cell bypass have been employed which require that redundant cells be added to the M2LC system of FIG. 3. The methods require the redundant cells to provide n+1 redundancy, n+2 redundancy, etc. by adding one additional cell row (rank), two additional cell rows, etc. to both the positive and negative arms of the output phase modules, and operating the redundant cells along with the normal number of required ranks under normal conditions. With these methods, when a particular cell fails in one arm (e.g., a cell in the positive arm of phase A), that cell is shorted by a switch (not shown), thereby placing the failed cell into a “0” state, and a complement cell (e.g., a cell in the negative arm of phase A) is placed in a “1” state to re-balance the voltages so that the sum of all cell voltages in a given output phase module equals the total DC link voltage. To keep the output voltages balanced, these methods duplicate the shorting of cells and placing a “1” state like wise in the other output phase module(s) (e.g., as required in the poles of B and C phase) so that the output line to line voltage is not affected by harmonics.
However, such methods suffer various shortcomings, two of which are described hereinbelow. First, placing a compensating “1” in the complement cell (the cell opposite the shorted cell) causes significant voltage ripple to occur in the individual DC links of the cells which contain the constant “1” state. The voltage ripple becomes significantly worse as more ranks are added to increase the cell redundancy level. Second, requiring all of the cells, both the normal number of cells required and the redundant cells, to operate under normal conditions causes a reduction in the efficiency of the M2LC system and an increase in the KVA rating of the M2LC system relative to what the efficiency and KVA rating would normally be if the minimum number of required cells were installed for the non cell bypass option.
Additional issues with known M2LC systems include acceptable operation at low output frequencies and the ability to develop sufficient DC output current. These performance features can be very important when the M2LC system is utilized for AC motor control, particularly for high starting torque applications. Since there is no external voltage source supplying the cell such as with the CCH topology, the output fundamental current should be fully maintained in the cells and their energy storage devices. Since it is well known that the impedance of a capacitor or electrical condenser monotonically increases with each decrease in output frequency, the resulting peak ripple voltage in the M2LC cell can exceed damaging levels at low frequencies under even rated current condition. Likewise, the ability of the M2LC system to produce DC current, which is important in starting brushless or synchronous motor applications, is difficult to attain with the M2LC system using known control techniques.